Mast cells are widely distributed throughout the different organs of mammals and are generally found in loose connective tissues. These cells are considered to be the origin of numerous mediators of immediate hypersensitivity. Preformed bioactive substances such as histamine, serotonin, and the peptide eosinophil chemotactic factor of anaphylaxis have been found to occur in mast cells. In the present study, immunoreactivity to selected regions of pro-ACTH/beta-endorphin was found in rodent-free mast cells in situ. Immunocytochemical localization was demonstrated for the NH2-terminal (16K) region of pro-ACTH/beta-encorphin, ACTH1-24, ACTH34-39, b beta-lipotropin 39-45 and b-endorphin. These findings suggest that mast cells contain a precursor with similar structural properties to that found in the pituitary gland. In addition, ACTH and beta-endorphin, as measured by radioimmunoassay, were released (0.1-0.5 micrograms/10 to the 6th power mast cells) concomitantly with histamine from free peritoneal and thoracic mast cells treated with compound 48/80, a known mast cell noncylotytic degranulating agent. Studies are in progress to confirm that peptides elaborated from mast cell pro-ACTH/beta-endorphin have biological activity.